Physical Volcanology of Komatiites and Ni-Cu-(PGE) Deposits of the Southern Abitibi Greenstone Belt

Abstract

Abstract Komatiitic rocks occur mainly in Archean greenstone belts, less commonly in Paleoproterozoic volcano-sedimentary belts, and only rarely in younger volcanic settings. As in most other greenstone belts worldwide, komatiitic rocks are locally abundant in the Abitibi greenstone belt but generally represent only a small proportion of the volcanic rocks in the volcanic succession. Although only locally exposed, glacially sculpted exposures of only weakly metamorphosed and mildly deformed komatiites of mineralized and unmineralized komatiites in the Abitibi greenstone belt are among the best in the world, characterized by excellent textural preservation and, in some cases, excellent mineralogical preservation. Komatiitic rocks in the Abitibi greenstone belt occur predominantly within the Pacaud (2750–2735 Ma), Stoughton-Roquemaure (2723–2720 Ma), Kidd-Munro (2720–2710 Ma), and Tisdale (2710–2704 Ma) assemblages, but have recently also been recognized in lesser abundances within the Deloro (2734–2724 Ma) and Porcupine (≤2690–≤2685 Ma) assemblages. Overall, the komatiitic rocks present in these assemblages are characterized by a wide variety of lithofacies (textural, compositional) and flow facies; however, a regional analysis of komatiite physical volcanology reveals some fundamental differences between each of the komatiite-bearing assemblages. The Kidd-Munro and Tisdale komatiite-bearing assemblages contain the largest volumes of komatiitic rocks, in particular thick, highly magnesian cumulate lava channels and channelized sheet flows. This suggests that the magma discharge rates were higher for these assemblages and/or that they formed more proximal to the eruptive site. However, the recently discovered Grasset Ni-Cu-(PGE) deposit hosted within relatively high MgO cumulate rocks that are interpreted to occur within the Deloro assemblage highlights the possibility of the other komatiite-bearing assemblages to contain similarly prospective volcanic and/or subvolcanic facies. Geochemical data indicate that regardless of age or petrogenetic affinity (Al-undepleted vs. Al-depleted vs. Ti-enriched vs. Fe-rich), almost all of the parental magmas were undersaturated in sulfide prior to emplacement and therefore represent favorable magma sources for Ni-Cu-(PGE) mineralization. Volcanological data indicate that almost all komatiite-associated Ni-Cu-(PGE) deposits in the Abitibi greenstone belt appear to be localized in lava channels or channelized sheet flows, which have the capacity to thermomechanically erode S-bearing country rocks and to efficiently transfer metals from the magma to sulfide xenomelts. Three type localities (Spinifex Ridge in La Motte Township, Pyke Hill in Munro Township, and Alexo in Dundonald Township) illustrate how physical volcanology (lava channelization) and stratigraphic environment (S source) need to operate quasi-simultaneously to allow for the genesis of significant amounts of Ni-Cu-(PGE) sulfides within a komatiitic succession. As not all komatiite magma pathways are mineralized, one of the most important challenges is to be able to distinguish potentially mineralized successions from barren successions.